As will be described in greater detail hereinafter, frequency divider methods and devices can be used to control frequency synthesizers operating in phase-locked loop (PLL-loop) circuitry in which the phase-locked loop circuit is provided for an oscillator.
Reference may be had to German Open Application DE-OS 35 44 371 and to the commonly owned U.S Pat. No. 4,697,156 which discloses a frequency synthesizer having an oscillator provided with a phase-locked loop utilizing fractional frequency divider control.
In German Open Application DE-OS 32 10 048, like German Open Application DE-OS 35 44 371, it is possible to obtain a fine frequency resolution in spite of a high keying frequency by a periodic adjustment of the frequency divider factor by +1
or by -1, for example so that the frequency divider which can be set only with integral (whole-number) frequency division factors m can have a frequency division effect which changes with time.
The requisite calculation of the frequency divider factor m in the case of the aforementioned patent and German Open Application 35 44 371, is accommodated utilizing a so-called phase accumulator while in German Open Application DE-OS 22 40 216, a similar effect is obtained by a synchronous divider chain. Upon overflow of the phase accumulator, to the whole number value of the frequency divider factor, +1 or -1 may be added.
In the German Patent Document DE-OS 22 40 216, the modification of the frequency divider factor by +1 (or by -1) is effected by a pulse-suppression circuit or by a pulse-injection circuit which is enabled upon each overflow of the synchronous divider chain to provide a single increment.
A frequency divider with periodic adjustment of the frequency divider factor m by +1 or -1 can also be termed a "fractional frequency divider" and a phase-locked loop which includes such a frequency divider can correspondingly be referred to as a "fractional synthesizer."
These systems will be discussed in greater detail below with reference to the drawing.
It suffices, at this point, to mention only the references previously described briefly and to observe that in the prior art devices, phase errors may be compensated with analog means. The effect of the compensation is, as a result, a function of the linearity and slope variations of the circuit elements which operate in the analog sense.